1. Field of the Invention
The present invention relates to a method for controlling the development in an automatic developing machine for a photosensitive lithographic printing plate precursor and automatic developing machine therefor and more particularly to a technique for minimizing fluctuation in developer sensitivity due to change in development processing conditions.
2. Description of the Related Art
In order to control the sensitivity of developer in the automatic developing machine for a photosensitive lithographic printing plate precursor, there is ordinarily employed a system of replenishing a development replenisher according to time lapse and processing amount to a developer (hereinafter referred to as an “area and time lapse-based replenishment system”), where the development replenisher is replenished with the passage of time to a developing tank storing the developer and at the same time, the development replenisher is replenished in an amount according to a value determined by measuring a plate area of the lithographic printing plate precursor processed.
However, this area and time lapse-based replenishment system is disadvantageous in that the developing unit of the automatic developing machine requires a high precision device for measuring the area of lithographic printing plate, causing the complication of configuration and the cost rise. This system is also disadvantageous in that it is difficult to judge to see which the lithographic printing plate precursor is photosensitive only on one side thereof or on both sides thereof and judge the kind of the printing plate precursor (e.g., printing plate precursors having different spreads of photosensitive layer composition). This raises a problem that when the required replenishment rate of developer replenisher changes with the change of the area and kind of lithographic printing plate precursor used and which the lithographic printing plate precursor is photosensitive only on one side thereof or on both sides thereof, it is made difficult to make proper replenishment of developer replenisher.
Under these circumstances, as the method of replenishing a development replenisher in the automatic developing machine for a photosensitive lithographic printing plate precursor, a system of replenishing a development replenisher to a developer (hereinafter referred to as an “electric conductivity-based replenishment system”) is known as described, for example, in Japanese Patent No. 2516022 (corresponding to JP-A-64-21451), where electric conductivity of the developer is measured, the measured value is compared with an electric conductivity value for providing an optimum sensitivity (hereinafter referred to an “adequate electric conductivity value”) that is experimentally determined previously, and the development replenisher is replenished when the measured value falls below the adequate electric conductivity value. The electric conductivity-based replenishment system ensures replenishment of an adequate amount of development replenisher to adequately maintain the sensitivity of developer, even with the change of the area and kind of lithographic printing plate precursor used and which the lithographic printing plate precursor is photosensitive only on one side thereof or on both sides thereof.
However, an experimental fact is generally known that a developer which has been fatigued only with carbon dioxide gas with time and a developer which has been fatigued only with plate processing differ from each other in adequate electric conductivity (silicate-based processing agent/fatigue with time (fatigue with carbon dioxide gas) at the time of replenishment restoration: 65 mS/cm; fatigue with processing at the time of replenishment restoration: 55 mS/cm; nonsilicate-based processing agent/fatigue with time at the time of replenishment restoration: 56 mS/cm; fatigue with processing at the time of replenishment restoration: 39 mS/cm). Therefore, in the aforementioned electric conductivity-based replenishment system, when the frequency of processing, e.g., amount processed per day differs from expected value, the ratio of actual time lapse replenishment rate to processing replenishment rate, too, differs from expected value. This causes the change of the adequate electric conductivity of the entire developer in the developing tank calculated from each adequate electric conductivity and hence the occurrence of some deviation from predetermined adequate electric conductivity, making it impossible to keep the sensitivity of developer adequate to disadvantage.
On the other hand, JP-A-9-96910 and JP-A-2004-219452, which is an improvement in JP-A-9-96910, each propose an automatic developing machine which measures the time interval of replenishment, determines the time lapse replenishment rate required within the time interval, regards the value obtained by subtracting the time lapse replenishment rate from the replenishment rate per one processing as processing replenishment rate to determine processing replenishment rate and time lapse replenishment rate and uses the electric conductivity calculated from the ratio of processing replenishment rate to time lapse replenishment rate thus determined to control the electric conductivity of developer. In accordance with this method, even with the change of the area and kind of the lithographic printing plate precursor to be processed and which the lithographic printing plate precursor is photosensitive only on one side thereof or on both sides thereof, the adequate electric conductivity of developer can be determined by estimating the ratio of processing replenishment rate to time lapse replenishment rate. In this method, however, the difference of required time lapse replenishment rate caused by the difference of developer fatigue with carbon dioxide gas (e.g., difference of developer temperature, presence or absence of liquid circulation) between during the operation of the automatic developing machine and during the suspension of the operation of the automatic developing machine cannot be detected, making it impossible to determine accurate ratio of processing replenishment rate to time lapse replenishment rate from time interval of replenishment across the suspension of the operation of the automatic developing machine. It is thus likely that some erroneous value of electric conductivity can be a reference value.
Under these circumstances, JP-A-2004-212681 proposes a method of calculating a reference value of electric conductivity using different calculation formulae from at the time of starting the automatic developing machine which is replenished with a required replenisher at the time of suspension thereof to during the operation of the automatic developing machine.